A search of the prior art has shown radiation curable coating compositions on a variety of substrates.
U.S. Pat. Nos. 4,070,262 and 4,071,425 to Guarino et al. and U.S. Pat. No. 4,072,592 to Due et al. all disclose radiation curable coatings including an adduct of an acrylic acid and epoxy resin and a reactive acrylate monomer. Adhesion of the UV cured coating to a substrate surface is improved by replacing a tertiary amine co-sensitizer at least in part with between 0.5 and 6% by weight of dimethylamino-ethyl acrylate. The coated substrates are preferably metal, but include paper, leather and cloth. In general the viscosities of the coatings range from 150 to 1200 cps. The presence of 0.1 to 6% of benzophenone as a photosensitizer in combination with 0 to 6% of a tertiary amine co-sensitizer is included in the compositions.
U.S. Pat. No. 4,072,770 to Ting discloses UV curable compositions comprising polyester-urethane polymers coated on substrates including metal, wood and floor tile. The photosensitizers used to cure the compositions include a combination of 2-chlorothioxanthone and a phenyl ketone (i.e. benzophenone).
U.S. Pat. No. 4,133,909 to Spencer discloses low viscosity radiation curable aqueous coatings including water soluble ethylenically unsaturated monomers. The presence of the monomer reduces the viscosity of the coatings and improves UV curing. Viscosities of the coatings are in the range of 55 to 230 cps. The coatings are aqueous based for spraying onto a substrate surface. Benzophenone in combination with a tertiary amino acrylate is included in the coatings as a UV photosensitizer.
U.S. Pat. No. 4,970,244 to Komai et al. discloses a photopolymerization initiator of compositions comprising epoxy-modified acrylic oligomers and di-, tri- and tetra- acrylates. The photoinitiator is present in the range of 0.01 to 10% by weight and includes a non-toxic benzophenone group containing peroxyester. Compositions are cured by exposure to UV wavelengths less than 420 nm.
U.S. Pat. No. 5,034,429 to Kaji et al. discloses photopolymerizable compositions including a polymerizable compound with a boiling point of 100.degree. C. or higher, a photoinitiator and a benzene derivative for producing a cured film for use in printed circuit boards. The photoinitiator may be a combination of an amine and benzophenone. The curing wavelength is at 250 to 550 nm.
Finally, U.S. Pat. No. 5,162,389 to Lee et al. discloses a composition cured by UV radiation in less than 1 second for coating on optical fibers. The composition includes a silicone component which rapidly cures when exposed to UV radiation. The composition includes monomethylester of hydroquinone (MEHQ) as a viscosity stabilizer. The composition viscosities are in the range of 1-20 Pa's (10 to 200 cps).
The prior art has shown that radiation curable coating compositions having radiation curable oligomers and monomers including acrylated epoxy oligomers and triacrylate monomers and photosensitizers including benzophenone and acrylated amines are known. In particular, the patents to Guarino et al. and Due disclose acrylic acid and epoxy resin oligomers and acrylate monomers. These composition coatings have low viscosities and are limited to coating metallic substrates. Spencer and Kaji disclose use of benzophenone and amines in combination as photosensitizers, but are directed to UV curing of either water soluble ethylenically unsaturated monomers or polymerizable compounds with boiling points over 100.degree. C. Ting and Lee are directed to different polymer composition components and Komai is directed to a specific type of benzophenone initiator. In general, all the compositions of the cited art have low viscosities, below 1200 cps. However, none of these references teach an ultraviolet curable coating composition including a combination of radiation curable oligomers, monomers and photosensitizers in a homogeneous mixture having a viscosity in the range of 4500 to 8000 cps. The present invention is directed to the provision of such an ultraviolet curable coating. It will be appreciated that advantages over known compositions are obtained by providing a highly viscous coating composition which virtually sits on top of a substrate surface, preferably a porous substrate, and when exposed to an ultraviolet light source results in curing of the coating composition onto the substrate surface without substantial penetration into the surface resulting in a gloss coated substrate.
Accordingly, it is a broad object of the invention to provide a radiation curable gloss coating composition and method of making the same.
A more specific object of the invention is to provide a high gloss ultraviolet curable coating composition comprising radiation curable oligomers and monomers and photosensitizers in a homogeneous mixture having a viscosity in the range of 4500 cps to 8000 cps, preferrably 4500 to 5500 cps.
Another more specific object of the invention is to provide a highly viscous coating composition which virtually sits on top of a substrate surface, preferably a porous substrate, and when exposed to an ultraviolet light source results in curing of the coating composition onto the substrate surface without substantial penetration into the surface.
Another object of the invention is to provide gloss coated products comprised of a substrate coated on at least one surface with the ultraviolet curable coating composition.
Another specific object of the invention is to provide a radiation curable coating composition used as a primer coat over a substrate surface wherein the coating surface is capable of printing with ultraviolet inks or ultraviolet compatible inks.
Another more specific object of the invention is to provide a radiation curable coating composition used, alternatively, or in addition to a primer coat, as a top coat over a substrate surface for imparting gloss characteristics, good rub resistance and flexibility to the substrate.
A further specific object of the invention is to provide an ultraviolet curable coating that is readily recycled with the coated substrate.